1. Field of the Invention
The present invention generally relates to a lens positioning structure and its method, particularly relates to a noncircular lens positioning structure and its method, and more particularly relates to the structure and method for making a plurality of centering axes of the noncircular lenses coincide with each other or incline to each other by a predetermined amount or shift to each other by a predetermined distance therebetween, and to the structure and its method for positioning the noncircular lens relative to its lens holding frame.
2. Description of the Related Arts
In case that a lens block including a noncircular lens and its lens holding frame is optically arranged with respect to another lens block including another noncircular lens and its lens holding frame so that a centering axis of the former lens does not coincide with a centering axis of the latter lens in an optical system, there have not been established any favorable techniques for coupling the lens blocks, which hold the noncircular lenses, with each other in a state, for example, that the centering axes of the noncircular lenses held by the lens holding frames are inclined to each other.
The noncircular lens is defined to be a lens which is not perfectly circular in circumferential profile and is not symmetrical with respect to its centering axis.
There have not been provided any conventional techniques in which such a noncircular lens is mounted on the lens holding frame.
In particular, when the centering axes of noncircular lenses are not coincident with each other, i.e., when the lenses are decentered lenses, the direction in which the optical path is refracted by the lens is not symmetrical with respect to the centering axis. Therefore, a rotation of the lens about the centering axis causes the optical path to change, making it impossible to construct an expected optical system.
Also, when a light flux restricting member, or so-called a stop (i.e. aperture, or diaphragm), that determines the effective optical path area between the noncircular lenses, is interposed therebetween, it is difficult to place the light flux restricting member between the lens blocks the centering axes of which are inclined to each other.
On the other hand, in a structure in which a lens having a circular periphery is held by the lens holding frame, the lens is held by a ring-shaped structure that fits to the entire periphery of the lens. Therefore, the lens is positioned relative to the frame with a high accuracy, i.e. a parallel shift in which a centering axis of one lens, and a centering axis of another lens, shift in parallel to each other in a direction perpendicular to a direction in which the centering axis exist, or an inclination in which a centering axis of one lens inclines with respect to a centering axis of another lens, is effectively prevented.
Also, a cut lens, a kind of noncircular lens, which is so shaped that a peripheral portion thereof is cut in parallel with the centering axis thereof, is positioned relative to its lens holding frame in which the outer peripheral portion of the cut lens is engaged with the inner peripheral portion of the lens holding frame over the mutual entire peripheries.
The lens holding frame with the entire-periphery fitting structure for a circular lens is not so difficult to manufacture due to the relatively simple construction. On the other hand, the entire-periphery fitting structure for the noncircular lens makes the structure itself of the lens frame complex and makes it difficult to position and hold the lens with respect to the lens holding frame accurately.
Further, there has not been established a technique for holding a plurality of noncircular lenses relative to their lens holding frame so that their respective centering axes are kept positioned to each other.